further fixes of py3 issues

complaints by pylint are mainly related to
- remove object from base list in class definitions
- unnecessary else/elif after return/raise

Change-Id: I13d15449149cc8bba0562338d0c9c42e97163bdf
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/21325
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop/gui/cfg_editor/widgets.py

@@ -104,8 +104,7 @@ class TreeWidget(QTreeWidget):
                 self.set_tree(read_config(self.file_path))
                 self.emit_save_status_changed(True)
                 return True
-            else:
-                self.file_path = None
+            self.file_path = None
         return self.new_tree()
 
     def new_tree(self):
@@ -391,7 +390,7 @@ class AddDialog(QDialog):
         if self.exec_() == QDialog.Accepted:
             if self.kind in [NODE, MODULE, INTERFACE]:
                 return [self.name.text(), self.get_value()]
-            elif self.kind in [PARAMETER, PROPERTY, COMMENT]:
+            if self.kind in [PARAMETER, PROPERTY, COMMENT]:
                 return [self.get_value()]
         return None